Since soft metals including stainless steel, iron base alloy, titanium, titanium alloy, aluminum, aluminum alloy, brass, copper, copper alloy, magnesium, and magnesium alloy are light in weight and inexpensive, these metals are worked into a wide variety of components for use in various fields. Examples of a soft metal article formed by processing the soft metals include decorative articles of wristwatch, such as a wristwatch case, wristwatch bezel, wristwatch case back, wristwatch band, clasp, crown, and so forth.
Meanwhile, the soft metals are inferior in terms of abrasion resistance because the surface hardness thereof is as low as not more than Hv=200 of Vickers hardness, thereby rendering the surface thereof susceptible to scratches. Further, softer metals such as iron base alloy, except stainless steel, titanium, and titanium alloy, aluminum, aluminum alloy, brass, copper, copper alloy, magnesium, magnesium alloy and so forth, have a property susceptible to corrosion because of their poor corrosion resistance. Accordingly, with the aim of improving the abrasion resistance and corrosion resistance of the soft metals used as the constituent material of soft metal articles, attempts have so far been made to apply various surface hardening treatments for hardening the surface of the soft metal articles to the surface thereof.
As methods of hardening the surface of the soft metal, there are generally available a method of forming a film coating the surface thereof, and a method of hardening the soft metal itself.
Examples of the method of forming a film on the surface of the soft metal include processes such as wet electroplating, ion plating, and so forth. As the wet electroplating, there are in widespread use nickel electroplating, nickel-phosphorus alloy electroplating, nickel-palladium alloy electroplating and so forth, however, an electroplating film formed by any of the processes described is soft and has not reached such a condition as to eliminate the risk of scratches being caused by impacts.
On the other hand, as the ion plating, there are cited processes of forming a film such as a hard carbon film, titanium nitride film, and so froth. However, since it is difficult to directly coat the soft metal, such as aluminum, aluminum alloy, brass, copper, copper alloy and so forth, with the hard carbon film, there is the need for forming the hard carbon film with an intermediate layer made of silicon, germanium, titanium and so forth, interposed therebetween. As a result, there exist more interfaces in this case due to the presence of the intermediate layer than in the case of directly coating the soft metal, resulting in a tendency that the hard carbon film is prone to peeling to that extent. Further, the titanium nitride film has poor adhesion property against the soft metal due to its high internal stress, so that the titanium nitride film has a property to be prone to peeling. Thus, with the ion plating, there has not been completely solved a problem that, whichever film is formed, the film is prone to peeling. Furthermore, there exists another problem that once the film, which is so important, is peeled, part of the soft metal itself is exposed, so that initiation of corrosion starts from that part, thereby rendering the soft metal unfit for use in the soft metal article.
As methods of directly hardening the soft metal itself, there have been known ion implantation, ion nitriding, gas nitriding, carburizing, and so forth. However, with any of those methods, since many hours are required in application of a hardening treatment, a soft metal article cannot be efficiently produced. In addition, with the use of any of those methods, grain coarsening of the soft metal results because of high temperature needed for the hardening treatment, thereby involving the risk of occurrence of surface roughness. For example, upon application of a gas nitriding to a soft metal article after applying a mirror surface polishing thereto, there occurs surface roughness of 200 to 300 μm due to the grain coarsening of the soft metal article after the gas nitriding, thereby causing a mirror surface to disappear, whereupon it becomes impossible to restore a mirror surface condition which has existed prior to the application of the gas nitriding even if a polishing is applied thereto thereafter, so that quality will be considerably impaired in terms of decorativeness of the external appearance. Especially for decorative articles of wristwatch, in particular, for decorative articles of wristwatch such as a wristwatch case, wristwatch bezel, wristwatch case back, and so forth, the quality in terms of decorativeness of the external appearance is an important element, and therefore deterioration in the quality in terms of decorativeness of the external appearance is a serious problem.
On the other hand, there is a case where those decorative articles of wristwatch are produced of an insulating material such as resin, ceramics, or the like besides the typical soft metals, and it is needless to say that the quality in terms of decorativeness of the external appearance is important also in this case so long as they are decorative articles of wristwatch. Accordingly, it is important that the surface of even decorative article of wristwatch produced of an insulating material is rendered insusceptible to scratches by enhancing abrasion resistance and is improved in the quality of decorativeness in terms of the external appearance by improving corrosion resistance so as to provide a high-class feeling to the external appearance of, especially a wristwatch, to be produced.
The invention has been developed to solve the problems described above, and it is an object of the invention to provide a soft metal and a method of producing the same, and a decorative article of wristwatch and a method of producing the same, wherein the surface thereof is rendered insusceptible to scratches by enhancing abrasion resistance and initiation of corrosion is prevented by improving corrosion resistance so as to provide excellent durability and a high-class feeling to the external appearance.